Jon Iverson spoke to Apple Tech support about the icon file in the trash problem. They said they were aware of it and planned to fix it in "the next or later update of Mac OS 8. They stressed that it is harmless and not to worry about it."
As to the larger issue of the proliferation of these files on your disk (not in the Trash), there is still no definitive explanation. In my conversations with Apple Tech Support, they suggested that these files were supposed to be there, that they were created by the system software in anticipation of being used as custom icon files and that this was documented somewhere in Inside Macintosh. When I asked why, if this is the case, they do not exist for all users on all drives in all folders, they had no explanation.
People continue to stress that having file sharing on and accessing a Mac across a network greatly increases the odds of these files appearing. The good news is that, in almost all cases, this remains primarily a minor problem. No loss of data or other serious problems occur.